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 CHAPTER IV.—INDIRECT RADIATORS.

The preceding chapter comprised mainly a discussion of the principles involved in the action of radiators in giving out their heat to the air and objects surrounding, but was confined almost entirely to direct radiation. Many of the deductions as to the relative value of different kinds of surface may be applied to indirect radiators as well, but a theoretical discussion of the latter requires some entirely different considerations from those presented in the last chapter on direct radiation. The indirect radiator is located below and outside of the room to be heated; it is enclosed by a casing, which has an air connection to the outside of the building, and a hot-air flue to the room to be heated. In this discussion it should be stated that the term indirect radiation is often applied to radiators or heating coils which are used in connection with a fan which creates a forced draft. In the author's opinion this is a mistake, as the element of forced draft involves still other considerations, and such radiators are merely heating coils for mechanical ventilation and should be discussed separately as such. The indirect radiator proper depends entirely upon the draft action of the heated column of air above it for its ventilating effect, and also for a means of communicating its heat to the room above.

Theory of indirect radiator.—The theory of the indirect radiator may be illustrated by the accompanying Figure 26, in which $$R\,$$ is the radiator set in a box, $$B\,$$, and provided with a cold-air connection, $$C\,$$, to the outside air (generally having a damper, $$d\,$$), and a hot-air duct, $$D\,$$, to the room to be heated, with a register, $$r\,$$, in the floor or wall of the room. Steam is supplied to the radiator by the pipe, $$p\,$$, through the casing. The heat of the radiator causes a column of hot air to rise through $$D\,$$, and the current is maintained by the excess of pressure of the column of cold air outside over that of the column of hot air in $$D\,$$. The exact pressure which creates this current is found in the excess weight of a column of cold air of height $$H\,$$ over that of a column of the same height and of the temperature of the air in $$D\,$$.

This may be calculated as follows, since the weight of a cubic